1. Field of the Invention
This invention relates to an improved process for removing acidic components from gaseous mixtures containing acidic components by the use of a salt solution comprising at least one sterically hindered substituted diamine. More particularly the invention relates to the conversion of a cyclic urea degradation product, which is formed from the sterically hindered amine, back to the original sterically hindered amine form.
2. Description of the Prior Art
It is well-known in the art to treat gases and liquids, such as mixtures containing acidic gases including CO.sub.2, H.sub.2 S, SO.sub.2, SO.sub.3, CS.sub.2, HCN, COS and oxygen and sulfur derivatives of C.sub.1 to C.sub.4 hydrocarbons with amine solutions to remove these acidic gases. The amine usually contacts the acidic gases and the liquids as an aqueous solution containing the amine in an absorber tower with the aqueous amine solution contacting the acidic fluid countercurrently.
The acid scrubbing processes known in the art can be generally broken into three categories.
The first category is generally referred to as the aqueous amine process where relatively large amounts of amine solution are employed during the absorption. This type of process is often utilized in the manufacture of ammonia where nearly complete removal of the acid gas, such as CO.sub.2, is required. It is also used in those instances where an acid gas, such as CO.sub.2, occurs with other acid gases or where the partial pressures of the CO.sub.2 and other gases are low.
A second category is generally referred to as the aqueous base scrubbing process or "hot potash" process. In this type of process an amine is included as an activator for the aqueous base used in the scrubbing solution. This type of process is generally used where bulk removal of an acid gas, such as CO.sub.2, is desired. This process also is applied to situations where the CO.sub.2 and feed gas pressures are high. In such processes, useful results are acheived using aqueous potassium carbonate solutions and an amine activator.
A third category is generally referred to as the non-aqueous solvent process. In this process, water is a minor constituent of the scrubbing solution and the amine is dissolved in the liquid phase containing the solvent. In this process, the amine comprises up to 50% of the liquid phase. This type of process is utilized for specialized applications where the partial pressure of CO.sub.2 is extremely high and/or where many acid gases are present, e.g. COS, H.sub.2 S, CH.sub.3 SH and CS.sub.2.
The present invention pertains to an improved process for practicing the second category of acid scrubbing process described above, namely, the aqueous base scrubbing process or "hot potash" process. Many industrial processes for removal of acid gases, such as CO.sub.2, use regenerable aqueous alkaline scrubbing solutions, such as an amine and potassium carbonate, which are continuously circulated between an absorption zone, where acid gases are absorbed and a regeneration zone, where they are desorbed, usually by steam-stripping. The capital cost of these acid scrubbing processes is generally controlled by the size of the absorption and regeneration towers, the size of the reboilers for generating stripping steam, and the size of the condensers which condense spent stripping steam so that condensate may be returned to the system to maintain proper water balance. The cost of operating such scrubbing plants is generally related to the amount of heat required for the removal of a given amount of acid gas, e.g., thermal efficiency, sometimes expressed as cubic feet of acid gas removed per pound of steam consumed. Means for reducing the costs in operating these industrial processes have focused on the use of absorbing systems or combinations of chemical absorbents which will operate more efficiently and effectively in acid gas scrubbing processes using existing equipment.
There are a number of patents which describe improvements to increase the efficiency of the "hot potash" process. Some of these improvements are described below.
U.S. Pat. No. 4,112,050, the disclosure of which is incorporated herein by reference, discloses the use of an aqueous solution comprising a basic alkali metal salt or hydroxide and a sterically hindered diamine activator for removing acid gases from a gaseous mixture. The sterically hindered amines are defined to include aminoethers, aminoalcohols, di- and triamines wherein the amino compounds contain at least one secondary amino group attached to either a secondary or tertiary carbon atom or a primary amino group attached to a tertiary carbon atom. U.S. Pat. No. 4,112,051 discloses the use of a sterically hindered diamine and a solvent for the amine, where the solvent is also an absorbent for the acidic gases. U.S. Pat. No. 4,112,052 is also directed at scrubbing solutions utilizing sterically hindered diamines. U.S. Pat. No. 4,094,957 is directed at the use of an aqueous solution comprising a basic salt, a sterically hindered diamine and an aminoacid cosolvent, the aminoacid cosolvent serving to prevent phase separation of the aqueous solution. U.S. Pat. No. 4,100,257 and 4,101,633 disclose the use of an amine mixture comprising a sterically hindered diamine and a tertiary amino alcohol for removal of acidic gases. U.S. Pat. No. 4,100,257 also discloses the use of the amine mixture in combination with a solvent which is also a physical absorbent for the acidic gases. These patents do not recognize that a portion of the diamine may be converted to a degradation product which is a cyclic urea, nor do these patents disclose a method for converting the cyclic urea back to its respective diamine. U.S. patent application Ser. No. 972,500 discloses that a cyclic urea degradation product may be formed from the sterically hindered diamine. However, this patent teaches that the cyclic urea should be removed from the solution by selective precipitation and filtration. This patent does not recognize that the cyclic urea can be reconverted back to the parent diamine while still in solution.
Dutch Pat. No. 78.12064 discloses a method for regenerating an aqueous scrubbing solution containing an amine which is contaminated by an oxazolidone. A fraction containing the oxazolidone is distilled off continuously.
U.S. Pat. No. 4,138,468 discloses a method for removing acidic gases such as CO.sub.2 and H.sub.2 S from a gaseous mixture by contacting the mixture with an alkanolamine. During this process some of the alkanolamine is converted to an oxazolidone, which is reconverted back to the alkanolamine by hydrolysis in a reactor maintained at 140.degree. to 200.degree. C. This process requires the addition of water to effect the hydrolysis and requires a minimum hydrolysis time of 48 hours. The water of the hydrolyzed liquid is separated after which the alkanolamine is separated from the oxazolidone.
In Imidazole and Its Derivatives Part I, Interscience Publishers (1953) at page 228, the hydrolysis of the cyclic urea, 2-imidazolidone, into ethylenediamine is disclosed, the author indicating, however, that the reaction requires a drastic hydrolysis with acid or alkali. In Ind. and Eng. Chemistry 40, 393 (1948) it is disclosed that 2-imidazolidone can be hydrolyzed with water at 200.degree. C. Zhur Obs Khim (1965) at 35, page 178 discloses that 1-methyl 2 imidazolidone can be hydrolyzed to N-methylethylenediamine by refluxing in a 20% NaOH solution.
It is desirable to provide a process for the conversion of undesired cyclic urea formed as a reaction product in an acid gas scrubbing solution which utilizes relatively moderate hydrolysis conditions, and is both continuous and capable of being utilized directly with conventional scrubbing equipment. It is also desirable to provide a process in which the hydrolysis can be effected in a relatively simple vessel in a relatively short time without the addition of other compounds to the scrubbing solution.
When certain sterically hindered diamines are present in a potassium carbonate solution and are contacted with CO.sub.2, the amine is partially converted to the carbamate form according to the following reaction: ##STR1## where R is a secondary or tertiary alkyl group and n is 2, 3 or 4.
It has been discovered that a portion of the activator may be converted to the corresponding cyclic urea. This reduces the performance of the scrubbing solution by decreasing the concentration of activator. This, in turn, inhibits the CO.sub.2 pick-up rate and also may lead to operational problems by forming two separate liquid phases. As described hereinafter, a method has been discovered for reducing the concentration of cyclic urea by converting at least a portion thereof back to its original diamine thereby improving the efficiency of the gas treating system.